The present invention relates to a video cassette recorder (VCR), and more particularly to a device for controlling head-drum angle and method therefor, for ensuring a clear screen condition by controlling the angle of a video head-drum in accordance with the driving speed of a video tape.
In general, image signals are recorded on the video tape while predetermined angles as shown in FIG. 2 are maintained. In the case of VHS video tape, the angle .theta. between the tape and the video head of the head-drum is given by the following expression (1): ##EQU1## where:
VT represents the driving speed of the video tape,
W represents the width of the video tape, measuring 10.07072139 millimeters,
D represents the diameter of the head-drum, measuring 62 millimeters,
.theta. represents the driving angle of the head at the instant of suspending, measuring 5.degree.56'7.38", and
fv represents the vertical synchronizing frequency, where 59.94 Hertz is used for NTSC-type broadcasting and 50 Hertz is used for PAL-type broadcasting.
Therefore, there is only one variable in the above expression (1). As shown in FIG. 2, the speed of the video head can be expressed as a vector A, whereas the driving speed of the video tape can be expressed as a vector B. If a compound vector expressed as a vector C is adjusted to be coincident with the direction of a channel, the driving angle .theta. of the video head can be directed to the vector C so that the information recorded in one pitch P of tape can be detected exactly.
If a user changes the video tape recorder to a high speed mode, the driving speed of the video tape VT increases, the result of this change is that the driving angle .theta., as expressed in equation (1), of the video head is also varied. FIG. 3 shows the change of the driving angle .theta. of the video head in relation to the speed VT of the video tape and represented by vector C.
When detecting the image signals from the video tape, the driving angle .theta. of the video head should be coincident with one pitch of the video tape in order to get a good quality image display. If these two are different from each other, as shown in FIG. 3B, the video head could not detect the image signals exactly and noise will occur on the display screen during high or low speed playback modes of operation of the VCR.
Thus various attempts have been made in the prior art to correct for the disadvantages described above. For example, as shown in FIG. 1, there is a DTF (Dynamic Tracking Follow) device which comprises a cylindrical permanent magnet winding coil through which D.C. current flows. The constitution and operation thereof will be described below.
As shown in FIG. 1D, a capacitor is provided D.C. voltage, mixed with an A.C. signal read out from a magnetic tape, can be removed. A video head can be moved in a direction to diminish the angle difference of tracking in order to get a good quality image display. However, the forgoing device can not cope with an abrupt variation in tape speed, i.e. greater than 10 times the current tape driving speed, because the variation of the video head speed is limited by the D.C. current.